Various optical devices (optical systems) have been proposed that are capable of forming a real image from a mirror image of an object to be projected that is placed to one side of an imaginary plane in space. The real image is formed in a position plane-symmetric to the object with respect to the imaginary plane. The inventor of the present invention has proposed a real mirror imaging device (hereafter called “dihedral corner reflector array” as necessary) which is an optical device including a large number of dihedral corner reflectors each including two specular surfaces (see Patent Reference No. 1) and a real mirror imaging optical system using a retroreflector array with retroreflection capability and a half-mirror (see Patent Reference No. 2).
The dihedral corner reflector array disclosed in Patent Reference No. 1, when an object (including physical objects and images) is placed to one of the sides of its optical device plane, will reflect light emitted from the above object by the pairs of specular surfaces of the dihedral corner reflectors while passing the reflected light through the optical device plane, thus forming a real image from mirror image of the above object in space at the observing side which is the other side across the optical device plane. The dihedral corner reflector array thus provides a novel way of optical imaging. The principle of operation of the dihedral corer reflector array is based on reflection of light by specular surfaces and can constitute a transmissive imaging device. By appropriately arranging multiple dihedral corner reflectors in one optical device plane, for example by directing the dihedral corner reflectors in one optical device plane to different directions, a real mirror imaging optical system can be constructed capable of forming a floating image that is observable from different directions by many observers (see Patent Reference No. 3). The real mirror imaging optical system disclosed in Patent Reference No. 2 has a novel way of optical imaging that effectively combines the property of the retroreflector to reflect incident light back in the incident direction with the property of the half-mirror to both reflect and transmit light, thereby enabling a floating image to be observed from a wide viewing angle.
Another real mirror imaging optical system has been proposed that makes use of reflection of light. The system uses an afocal lens array including an afocal optical system with infinite focal distance to form an image of an object to be projected in a position in space that is symmetric to the object with respect to the optical device plane of the afocal lens device (see Patent Reference No. 4).    Patent Reference No. 1: WO 2007/116639    Patent Reference No. 2: JP 2009-025776    Patent Reference No. 3: WO 2008/111426    Patent Reference No. 4: JP 2005-010755